Microwave ferrite isolator having a circumferential biasing magnetic field applied to a cruciform waveguide section



United States Patent Ofilice 3,3fi4,52l Eatentedl Feb. 14, 1967 MICRQWAVE FERRITE ISOLATQR HAVING A CIRCUMFERENTIAL BIASING MAGNETIC FllELD APPLIED TO A CRUCHFURM WAVE- GUIDE SECTION Elmer Freibergs, Long Branch, Nathan Lipetz, Oakhurst, and Richard A. Stern, Eatnntown, N.J., assignors to the United States of America as represented by the Secretary of the Army Filed May 10, 1965, Ser. No. 454,755 Claims. (Cl. 33324.2)

The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to microwave isolators and more particularly to waveguide type isolators adapted for use with circular waveguide modes.

In view of its low loss and high power handling characteristics, circular waveguides propagating the TE mode have proved to be highly desirable as transmission mediums in microwave systems, especially in the millimeter wave region. Although it is well known that ferrites may be utilized in rectangular waveguides to provide nonreciprocal propagation at ferromagnetic resonance, this approach has been found extremely difficult for circular waveguides. This is due to the fact that the magnetic field configuration in circular waveguides is such that a ferrite ring positioned concentrically within the waveguide must be circumferentially magnetized in order to achieve non-reciprocity at resonance. Inasmuch as this requires a ferrite cylinder having a very narrow wall thickness, several thousandths of an inch for example, it has been found extremely difficult to fabricate a ferrite cylinder having such an extremely thin wall thickness.

It is an object of the present invention to provide a mil limeter wave isolator which overcomes the aforementioned limitations.

It is an object of the present invention to provide a millimeter wave isolator which overcomes the aforementioned limitations.

It is another object of the present invention to provide an improved isolator for use in "IB circular mode waveguide systems operating at millimeter frequencies which is compact, light weight and has good electrical characteristics.

In accordance with the present invention, there is provided a microwave isolator which includes first and second cylindrical waveguide sections and a cruciform waveguide section having four channels intermediate the cylindrical waveguide sections and longitudinally aligned therewith. Each channel of the cruciform waveguide section comprises a relatively narrow end wall and two relatively wide walls. Also included are respective ferrites mounted within each of the channels and positioned close to the respective narrow end walls of each of the channels but spaced therefrom. Included further are magnetic biasing means mounted in abutment with the outer surface of the side walls for producing a circumferential magnetic field which encompasses the waveguide channels in the vicinity of the ferrites.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings in which:

FIG. 1 illustrates an embodiment of the present invention;

FIG. 2 is a plan view, partially in section, taken along the lines 2-2 of FIG. 1;

FIG. 3 is a cross section taken along the lines 3-3 of FIG. 1; and

FIG. 4 illustrates the electric field distribution in the cruciform waveguide section shown in FIG. 3.

FIGURE 5 shows the details of the substrate and ferrite slab.

Referring now to FIGS. 1-3 of the drawings, there is shown at 10 a hollow waveguide of a cruciform shaped cross section positioned between two sections of identically dimensioned cylindrical waveguides 12 and 14- which are coaxially aligned and are adapted to propagate the TE mode. The cruciform waveguide 10 comprises four U-shaped channels 11, 13, 15 and 17 which extend longitudinally along the common axis of cylindrical waveguides 12 and 14 and symmetrically arranged with respect to the common axis of the oppositely disposed cylindrical waveguides 12 and 14. The ends of the cruciform waveguides are transformed into the respective cylindrical waveguide sections by means of conventional transition waveguide sections as shown at 16 and 18. As shown, each U-shaped channel of the cruciform waveguide 10 is terminated by longitudinally extending end walls as at 20 which are narrower than the associated channel side Walls 22. Within each of the channels 11-17 there is positioned a ferrite slab 24 which is mounted on a dielectric substrate 26. The details of the substrate 26 and ferrite slab 24 are shown in FIG. 5. Each of the ferrite slabs 24 are identical in dimension and composition and are comprised of hexagonal ferrite materials which possess high anisotropy fields and are well known to those skilled in the art. As shown, the substrates 26 are tapered at both ends for better matching purposes. The respective substrate mounted ferrite slabs 24 are positioned substantially at the longitudinal center of each of the cruciform waveguide channels 11-17 with the exposed surface of the respective ferrite slabs 24 facing respective narrow end walls 20 of each of the cruciform waveguide channels and closely spaced therefrom. The spacing between the respective ferrite slabs 24 and associated end walls 20 of each of the cruciform waveguide channels 11-17 is made uniform to maintain symmetry. The tapered ends of the ferrite slab supporting substrates 26 will of course extend longitudinally towards the cylindrical waveguide sections 12 and 14. Thus, as shown in FIG. 3, each of the cruciform waveguide channels 11-17 has positioned therein a substrate mounted ferrite slab 24 positioned between the wide walls 22 with the exposed ferrite surface close to and facing the end walls 20. For non-reciprocal absorption, the ferrite slabs 24 must be positioned where the RF magnetic field is circularly polarized in each of the four waveguide channels 11-17. The optimum position of the ferrite slabs 24 is relatively close to the end walls 20 and may be determined empirically. The optimum dimensions of the ferrite and dielectric substrates, in order to obtain maximum reverse loss with minimum forward attenuation, may also be determined empirically. To prevent any fringing of the E field between the wide wall interfaces of each of the waveguide channels 11-17, the height of the ferrite slabs was made equal to the spacing between the wide wall 22 interfaces. The dielectric substrates 26 were made thick enough to give a substantial increase in reverse loss without degrading the forward loss, and long enough to accommodate the ferrite slabs 24. As shown in FIGS. 1 and 3, ferrite biasing magnetic fields are provided along the outer walls of the cruciform waveguide channels. Permanent magnets as at 30 are arranged along the outer surfaces of wide walls 22 of each of the channels 11-17 such that each of the ferrite slabs 24 is provided with an equal bias and, also, there is provided a continuous circular magnetic bias path which effectively encompasses the cruciform waveguide channels in the vicinity of the ferrite slabs 24. To accomplish this, each magnet labeled H in FIG. 3 complements a similarly constructed magnet H along the outer surface of the wide walls 22 of the cruciform waveguide channels 11-17. The direction of each of the biasing magnetic fields is indicated by the arrows. Since the hexagonal ferrite possesses a high anisotropy field, the size of the magnets 30 are relatively small in size.

In discussing the operation of the isolator, it is to be assumed that the RF TE mode energy is to be propagated Without loss in the direction of the arrow A while the RF TE mode energy in the opposite direction, arrow B, is to be attenuated. With the cylindrical TE mode propagated through input cylindrical waveguide 12, the circular electric field in the cylindrical Waveguide is propagated through the cruciform waveguide channels 11-17 with a transverse E field distribution in each of the four channels which is quite similar to that of the TB dominant mode in rectangular Waveguide. This field distribution is shown in FIG. 4. As in the case of the TE mode in rectangular waveguide, two regions of RF circular polarization due to the propagated wave also exists in each of the channels 11-17 of cruciform waveguide 10. One of these regions is closer to the central axis of the cruciform waveguide while the other is closer to the channel end walls 26. The region more distant from the center axis is utilized for the placement of the ferrite slabs 24 inasmuch as it offers a convenient place for the external biasing means 30 and also avoids any undesirable field fringing otherwise present with the ferrite slabs positioned close together near the cruciform waveguide axis. With the ferrite slabs 24 so positioned in their respective waveguide channels, they act in the normal manner as in rectangular waveguides to provide isolator action. As the propagated RF energy passes out of cruciform waveguide 10, the transverse E field distribution transforms again to a pattern similar to that of the circular guide TE mode.

It was found that the isolator hereinabove described provided reverse to forward db attenuation ratios in excess of 15.1 over an 8% bandwidth centered at 35.22 gc.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various 4 changes and modifications may be made therein without departing from the invention, and it is therefore aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A microwave isolator comprising first and second cylindrical waveguide sections, a cruciform waveguide section having four channels intermediate said cylindrical waveguide sections and longitudinally aligned therewith and coupled thereto, each channel of said cruciform waveguide section comprising a relatively narrow end wall and two relatively wide side walls, respective ferrites mounted within each of said channels and positioned close to the respective narrow end walls of each of said channels but spaced therefrom, and magnetic biasing means mounted in abutment with the outer surfaces of said side walls for producing a circumferential magnetic field encompassing said waveguide channels in the vicinity of said ferrites.

2. The isolator in accordance with claim 1, wherein each of said ferrites comprise a ferrite slab mounted on a substrate, with the exposed surface of said respective ferrite slabs facing the respective narrow end walls of said channels.

3. The isolator in accordance with claim 2 wherein the height of the exposed surfaces of the respective ferrite slabs are equal to the spacing between associated wide wall interfaces.

4. The isolator in accordance with claim 3, wherein said ferrite slabs are longitudinally centered within their respective channels.

5. The isolator in accordance with claim 3, wherein said ferrite slabs comprise hexagonal ferrite materials characterized by relatively high anisotropy fields.

No references cited.

HERMAN KARL SAALBACH, Primary Examiner.

P. L. GENSLER, Assistant Examiner. 

1. A MICROWAVE ISOLATOR COMPRISING FIRST AND SECOND CYLINDRICAL WAVEGUIDE SECTIONS, A CRUCIFORM WAVEGUIDE SECTION HAVING FOUR CHANNELS INTERMEDIATE SAID CYLINDRICAL WAVEGUIDE SECTIONS AND LONGITUDINALLY ALIGNED THEREWITH AND COUPLED THERETO, EACH CHANNEL OF SAID CRUCIFORM WAVEGUIDE SECTION COMPRISING A RELATIVELY NARROW END WALL AND TWO RELATIVELY WIDE SIDE WALLS, RESPECTIVE FERRITES MOUNTED WITHIN EACH OF SAID CHANNELS AND POSITIONED CLOSE TO THE RESPECTIVE NARROW END WALLS OF EACH OF SAID CHANNELS BUT 